The copying and redistribution of commercial imagery, audio and video productions has long been a cause of lost revenues to the creators/producers of such material. The advance of technology has not only expanded the means of legitimate distribution for audio and visual/video works, but has also made it easier to copy these materials for unauthorized purposes. In addition, there is a need to monitor, verify and track broadcasts and other distributions of this content.
Various methods have been developed to eliminate or limit both sophisticated and unsophisticated illegitimate distribution as well as to monitor distribution. Some of these methods rely on physical means. Others employ an auxiliary embedded signal to control, monitor and track usage of the content.
In accordance with preferred embodiments, a multi-bit auxiliary message (sometimes termed a “digital watermark”) is robustly embedded directly into the audio signal. Hardware or software systems can then read this message and, for example, and control, monitor or track usage of the content.
Key practical issues are addressed whereby the perceptual impact of this added message can be adjusted—both overall and as a function of the underlying audio content. In addition, robustness to various forms of distortion are addressed in a manner superior to the prior art.
One aspect of the invention is a method of embedding data in an audio signal. The method provides a message comprising message bits, and associates carrier signals with the message bits. The method modulates the carrier signals according to values of the associated message bits. The modulating operation changes a property of the carrier signal to correspond with a desired value of the associated message bit. For example, in one embodiment, the modulating changes the polarity of the carrier signal. The method adjusts the modulated carrier signals as a function of a perceptibility threshold computed from the audio signal where the carrier signals are to be embedded. Finally, the method combines the adjusted carrier signals with the audio signal in a manner that yields an audio signal that is perceptually similar yet includes the embedded message data.
There are a variety of alternatives and enhancements to this method.
Additionally, there are a variety of methods for detecting the embedded data, and enhancements for improving detection, even where the audio signal has been subjected to distortion or is a compressed audio signal.
Another aspect of the invention is a method of detecting data embedded in an audio signal. This method applies a carrier signal to the audio signal to produce an output signal including peaks corresponding to embedding locations of the carrier signal. The process of applying the carrier may include, for example, re-combining the carrier through a correlation or other signal detection operation. The method analyzes the output signal to perform a registration of the embedding locations in the output signal. The method determines message values from the peaks at the embedding locations in the output signal. Further enhancements and variants of these methods are described below.